Powder coatings are widely used for the general painting of metals, as environmentally friendly paints which do not release organic solvents into the atmosphere during paint application.
Normally, a powder coating is produced by producing a main constituent resin, subjecting the resin to coarse crushing, and dry blending with a curing agent and various additives, and then subjecting the dry blend to extrusion mixing, cooling, a second crushing process, and classification. Compared with solvent based paints, the production process is long and complex, and consequently the problem arises of increased production cost. In order to resolve this problem, tests are being conducted on preparing a raw materials solution by dissolving or dispersing the raw materials, namely the main constituent resin, the curing agent and the various additives, in a solvent, and then removing the solvent to obtain the powder coating directly.
In such a case, in order to obtain the final product powder coating, a step for removing the solvent is necessary, although if during solvent removal the raw materials solution is heated to a high temperature, the main constituent resin and the curing agent undergo a curing reaction, resulting in gelling.
In contrast in Japanese Unexamined Patent Application, First Publication No. Hei 10-53729, a method is disclosed wherein a powder coating is obtained by using a specific continuous reduced pressure solvent removal apparatus to remove the solvent at a comparatively low temperature. However, the powder coating produced is lumpy, and in order to use the product as a paint, a fine grinding and classification process is necessary. Furthermore, there is also a possibility of localized heating inside the apparatus causing gelling during the solvent removal.
Methods of removing solvent from a powder coating raw materials solution by using a supercritical fluid have also been proposed (Japanese Unexamined Patent Application, First Publication No. Hei 8-113652 and Published Japanese Translation No. Hei 8-503721 of PCT International Publication), although in these methods, operations need to be performed under high pressure conditions, and so a problem arises in that the equipment required for production is extremely expensive, making the methods inappropriate for industrial production.
Another method for removing solvent is disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 9-255895 and is a method for obtaining a powder coating by using a freeze drying method, but once again, problems arise with this method in that the energy costs are very high and the productivity low.
In addition to these methods, proposals have also been made which utilize spray drying for the removal of solvent. For example in Japanese Unexamined Patent Application, First Publication No. Hei 3-192128, a method is disclosed wherein a powder coating raw materials solution dispersed in water is spray dried to produce a powder. However in the case of this method, because a dispersant or the like must be used in order to disperse the powder coating raw materials solution in water, not only does the cost of the raw materials increase, but there is also a danger of a reduction in the storage stability of the paint, and reductions in the water resistance and chemical resistance of the formed paint film. Furthermore, because water, which has a large specific heat and a large latent heat of vaporization, is used as the medium, the thermal energy cost required in the drying process is also extremely high.
In contrast, in Japanese Unexamined Patent Application, First Publication No. Sho 63-267402, a method is disclosed in which a raw materials solution comprising a main constituent resin and a solvent is subjected to drying. However, in the case of this production method, because a high temperature solvent gas which has been heated to a superheated vapor state is used as the heat source gas, there is a danger that within the system comprising the main constituent resin and the curing agent, the powder coating raw materials will be exposed to high temperatures during the drying process, causing the main constituent resin and the curing agent to undergo a curing reaction which will result in a gelling of the paint, making it unusable as a powder coating.
Furthermore, generally a problem exists in spray drying methods in that hollow particles are easily generated, and so a problem of powder coatings that have been produced by spray drying is that, depending on the paint application conditions, bubbles, pinholes and the like may occur in the paint film.
In addition, in powder coatings produced by conventional mechanical crushing methods, the production method means that the mixing of the main constituent resin and the curing agent is always insufficient, and so the external appearance of the paint film is inferior when compared with solvent based paints. As a result, the use of powder coatings in applications requiring a superior paint film external appearance, including the use on automobiles, has been problematic.